Reverse in situ combustion and ignition of carbonaceous strata



June 13, 1967 B ET AL 3,324,945

REVERSE IN SITU COMBUSTION AND IGNITION OF CARBONACEOUS STRATA Filed Oct. 26, 1964 INJECTION INVENTORIS' VV.B. LUMPKIN RE. GILCHRIST United States Patent O 3,324,45 REVERSE IN SITU IOMBUSTION AND IGNITION OF QARBONAEOUS STRATA Wiiliam B. Lumpkin and Ralph E. Gilchrist, Barilesville,

Okla, assignors to Phiiiips Petroleum Company, a corporation of Delaware Filed Oct. 26, 1954, Ser. No. 406,252 10 Claims. (#31. 166-411) This invention relates to an improved process for the reverse ignition and/or in situ combustion of a subterranean petroliferous stratum.

The recovery of hydrocarbons from oil-bearing strata by in situ combustion is a conventional process in the oil industry. Depending upon the condition of the stratum to be produced, either a forward or reverse burning combustion zone is utilized in the production process. In strata in which forward drive of the combustion zone results in plugging of the stratum in advance of the combustion zone, a reverse burning front is usually utilized to avoid the plugging aspect of the forward burning operation. In the usual method of carrying out a reverse burning project, as the pressure in the formation increases, oil recovery decreases, resulting in high injected air-toproduced oil ratios. This results in an increased cost per barrel of oil produced. High injected air-to-produced oil ratios result in higher combustion temperatures and consumption of greater proportions of oil than would result at lower temperatures.

This invention is concerned with a reverse burning process which reduces combustion temperature in the stratum and improves oil recovery.

Accordingly, it is an object of the invention to provide an improved process for igniting an oil-bearing stratum by reverse injection of combustion-supporting gas, such as air, oxygen-enriched air, or any gas containing suflicient free oxygen to sustain combustion. Another object is to provide an improved process for producing hydrocarbons from an oil-bearing stratum by reverse in situ combustion. A futher object is to reduce the ignition and/ or combustion temperature in a reverse burning in situ combustion process as applied to an oil reservoir. Other objects of the invention will become apparent upon consideration of the accompanying disclosure.

A broad aspect of the invention as applied to recovery of oil from an oil-bearing stratum penetrated by an injection -well and a production well comprises injecting a combustion-supporting gas into the injection Well at a sufiicient rate to sustain combustion within the stratum, injecting into the injection well a slug of liquid hydrocarbon having a vapor pressure in the range of 0.1 to 15 percent of the ambient stratum pressure under operating conditions and an ignition point in the range of about 300 to 1000 F, and igniting the stratum adjacent the production well so that the combustion-supporting gas contains vaporized liquid hydrocarbon from the injected slug in the range of about 1 to 5 volume percent. The flow of injected gas and hydrocarbon vapor may precede the ignition around the production well so that the hydrocarbon vapor in the combustion-supporting gas serves as ignition fuel. It is also feasible to ignite the stratum adjacent the production well prior to the injection of liquid hydrocarbon into the injection well in which case the injected liquid supplies vaporous hydrocarbon of lower ignition point to the combustion zone and assists in maintaining the combustion zone temperature lower than this temperature would be without the injection of the liquid hydrocarbon. The liquid hydrocarbon material may be injected in small concentrations continuously or in small slugs intermittently. At any rate the liquid exists as liquid under ambient stratum conditions. The size of the liquid slug is determined by reservoir conditions. In any event,

3,324,945 Patented June 13, 1967 the injection of excessive amounts of liquid hydrocarbon which might result in the driving of a liquid front toward the combustion zone must be avoided. Usually, the amount of liquid hydrocarbon injected in a slug is in the range of 0.1 to 1.0 percent pore volume.

Illustrative fuels which may be used at different reservoir temperatures and pressures are shown in the tables below. Table I shows three reservoir conditions as to temperature and pressure along with the vapor pressure range in -p.s.i.a. within the range of 0.1 to 15 percent of the ambient pressure in the reservoir. Table II illustrates 3 different fuels which may be used in the reservoir illustrated in Table I.

TABLE I.RESERVOIR CONDITION SHOWING VAPOR PRESSURE RANGES AT THREE DIFFERENT PRESSURES Temperature F.) Pressure (p.s.i.a.) .1 to 15% of Vapor Pressure Range (p.s.i.a)

TABLE II.POSSIBLE FUELS FOR USE UNDER RESERVOIR CONDITIONS OF TABLE I Vapor Pressure at Ignition Temperature, Fuel Reservoir Conditions F. of Fuel (p.s.i.a.))

(D) n-Butane 806 (E) n-Pentane 37 588 (F) n-Hexane 13 488 It should be noted that fuel (D) is not usable under reservoir conditions (A) but is operable under reservoir conditions (B) and (C). Fuels (E) and (F) are operable under reservoir conditions (A), (B), and (C). The liquid fuel to be added can readily be determined from a knowledge of the vapor pressure of the various available fuels and from a knowledge of the specific reservoir conditions in the stratum to be produced. The liquid hydrocarbon obviously need not be a single, pure hydrocarbon, but may be a mixture of hydrocarbons having vapor pressure and ignition temperature characteristics which are operable in the reservoir or stratum to be ignited and burned. Thus, it can be seen that a mixture of n-butane and npentane, with suflicient n-pentane in the mixture to lower the vapor pressure to the range illustrated in (A) of Table I would be useful in operating in such a reservoir.

Other mixtures of the three hydrocarbons and also other.

mixtures of hydrocarbons which have the necessary vapor pressure and ignition temperature are readily operable in the process. Gasoline boiling range hydrocarbons having relatively high vapor pressure function Well, particularly a gasoline having a substantial concentration of natural gasoline hydrocarbons therein. Stoddard solvent, Sotrol, naphtha, and similar hydrocarbon mixtures having the necessary vapor pressure and ignition temperature are also operable in the process. The ignition temperature of the liquid hydrocarbon is in the range of 3001000 F. and, preferably, in the range of 300825 'F.

The injection of liquid fuel into the injection well results in providing a concentration of the vapor of the fuel in the air stream entering the combustion zone in the range of about 1 to 5 volume percent of the air stream. This results in three improvements in the in situ combustion process, viz., (1) a lowering of the peak temperature in the combustion zone, (2) narrowing of the combustion zone (along the line of flow of fluid resulting in a thinner combustion zone), and (3) an increased efiiciency in oxygen utilization. These three advantages result in greater oil recovery and a decreased injected air-to-produced oil ratio due to decreased coking and cracking in the combus- 3 tion zone. When applied to the ignition step of the process, it results in substantially lower ignition temperature and more rapid ignition than would otherwise occur.

The invention thus overcomes some of the objectionable features of reverse burning in producing by in situ combustion, such as excess use of oxygen which causes increased temperatures. The increase in temperatures results when excess oil is burned and this oil should be produced. Less cracking of the oil occurs when burning is effected at lower temperatures, thereby saving oil normally burned.

While the invention is intended primarily for application at reservoir pressures of at least 500 p.s.i, it is operable at pressures of less than 500 p.s.i.

A more complete understanding of the invention may be had by reference to the accompanying schematic drawing which is a plan view thru an oil-bearing stratum penetrated by an injection well and a production well illustrating operation in accordance with the invention.

Referring to the drawing an oil-bearing stratum 10 is penetrated by an injection well 12 and a production well 14. It is to be understood that production well 14 may represent a central well surrounded by a ring of wells 12 or that it may represent a line of production wells in between a line of injection wells 12 on each side thereof.

Wells 12 and 14 are normally provided with a casing extending to stratum 10 or thru stratum 10 with perforations therein allowing for injection and production of fluids. Tubing strings 16 and 18 are positioned in well 12 and 14, respectively. Lines 20 and 22 are auxiliary lines connecting with wells 12 and 14, respectively, for injection and withdrawal of fluids from the well as necessary.

At a certain stage of operation, a combustion zone 24 has been moved into the stratum from production well 14 by injecting a combustion-supporting gas (air) thru injection well 12 either thru line 20 or directly into tubing string 16. A slug of liquid hydrocarbon 26 has been injected into the stratum adjacent well 12 by injection either thru line 20 or directly into tubing string 16. This injection of liquid hydrocarbon initially fills a lower section of the well bore and is driven into the stratum with continued injection of air.

When utilizing the technique of the invention in initiating combustion around well 14, air is injected thru well 12 until air is produced in well 14 at a sufficient rate to sustain combustion and a slug of liquid hydrocarbon suitable for use in the process in accordance with reservoir conditions is injected thru well 12 into the surrounding stratum so that vapor of the liquid hydrocarbon in a concentration in the range of 1 to percent is in the air reaching well 14. At this time the temperature of the well bore within stratum is raised to ignition temperature so that the oil and vaporous hydrocarbon burn in the combustion area. It is feasible to preheat the borehole wall of well 14 substantially to combustion-supporting temperature in the presence of the injected air-fuel mixture so that combustion takes place as the air-fuel mixture arrives at the hot stratum surrounding well 14. In this manner combustion takes place substantially immediately and the resulting combustion zone is moved toward injection well 12 with production of hydrocarbons thru tubing string 18. Intermittent or continuous injection of liquid hydrocarbon into the stratum thru well 12, as required to maintain a desired concentration of vaporous fuel in the injected air is practiced.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

We claim:

1. In a process for producing hydrocarbons from a previously unburned subterranean petroliferous stratum penetrated by an injection well and a production well by in situ combustion involving igniting said stratum adjacent said production well to establish a burning zone, feeding combustion-supporting O -containing gas thru said stratum from said injection well to move a reverse burning front toward said injection well, and recovering production thru said production well, the improvement comprising the steps of:

(a) depositing a liquid hydrocarbon in said stratum only adjacent said injection well, said hydrocarbon having a vapor pressure in the range of about 0.1 to 15 percent of ambient stratum pressure and an ignition temperature in the range of about 300-825" F.;

(b) passing the injected combustion-supporting gas in contact with said liquid hydrocarbon so as to entrain vapor of said liquid hydrocarbon in a concentration in the range of 1 to 5 volume percent of the injected gas and move the resulting gaseous mixture thru the stratum from the deposited liquid to said burning zone; and

(c) burning the vapor of step (b) in said burning zone at a lower temperature than results from operation without said vapor.

2. The process of claim 1 wherein said liquid hydrocarbon is injected as a slug amounting to 0.1 to 1.0 pore volume.

3. The process of claim 1 wherein the liquid hydrocarbon in step (a) is deposited in the stratum prior to ignition at said production well and step (b) is initiated sufiiciently prior to said ignition to supply said vapor for ignition, thereby lowering normal ignition temperature.

4. The process of claim 1 wherein the liquid hydrocarbon is a gasoline fraction.

5. A process for initiating in situ combustion in a previously unburned oil-bearing stratum penetrated by an injection well and a production well which comprises the steps of:

(a) passing combustion-supporting, O -containing gas thru said stratum from said injection well to said production well;

(b) thereafter, depositing a liquid hydrobaron in the stratum only adjacent said injection well having a vapor pressure in the range of 0.1 to 15 percent of ambient stratum pressure under operating conditions and an ignition point in the range of 300 to 1000 P. so that vapor from said liquid is passed with the injected gas in step (a) thru said stratum from the dcposited liquid to the production well; and

(c) igniting the stratum adjacent said production well so that said vapor is burned as a portion of the fuel of combustion.

6. The process of claim 5 wherein the ignition step of (c) is effected after step (b) with said vapor available as fuel of combustion.

7. The process of claim 5 wherein the vapor of said liquid hydrocarbon is in the range of about 1 to 5 volume percent of the injected gas.

8. A process for recovering oil from a previously unburned oil-bearing stratum penetrated by an ignition well and a production well by reverse in situ combustion which comprises the steps of:

(a) passing combustion-supporting, O -containing gas thru said stratum from said injection well to said production well;

(b) thereafter, depositing a liquid hydrocarbon in the stratum adjacent said injection well having a vapor pressure in the range of 0.1 to 15 percent of ambient stratum pressure under operating conditions and an ignition point in the range of 300 to 1000 P. so that vapor from said liquid is passed with the injected gas in step (a) to the production well;

(c) igniting the stratum adjacent said production well so that said vapor is burned as a portion of the fuel of combustion at a lower combustion temperature than produced by burning stratum oil alone;

(d) continuing the How of gas as in step (a) following ignition so as to move the resulting combustion zone toward said injection well; and

(e) recovering produced fluids thru said production References Cited well.

9. The process of claim 8 wherein slugs of said liquid UNITED STATES PATENTS hydrocarbon are periodically injected into the injection 3,062,283 11/1962 Trantham 166-41 Well as in step (b) so as to maintain said vapor in the 5 3,072,186 1/1963 pafker 166 11 injected gas and substantially depress the combustion tem- 3,113,619 12/1963 Relchle 166 '11 perature in said combustion zone. 33671121 1/1965 Sharp 166 11 10. The process of claim 8 wherein the vapor of said liquid hydrocarbon is in the range of about 1 to 5 volume CHARLES O CONNELL Pr'mary Examiner percent of the injected gas. STEPHEN NOVOSAD, Examiner. 

5. A PROCESS FOR INITIATING IN SITU COMBUSTION IN A PREVIOUSLY UNBURNED OIL-BEARING STRATUM PENETRATED BY AN INJECTION WELL AND A PRODUCTION WELL WHICH COMPRISES THE STEPS OF: (A) PASSING COMBUSTION-SUPPORTING, O2-CONTAINING GAS THRU SAID STRATUM FROM SAID INJECTION WELL TO SAID PRODUCTION WELL; (B) THEREAFTER, DEPOSITING A LIQUID HYDROCARBON IN THE STRATUM ONLY ADJACENT SAID INJECTION WELL HAVING A VAPOR PRESSURE IN THE RANGE OF 0.1 TO 15 PERCENT OF 